Ethane reacts with chlorine to form chloroethane - CIE - A-Level Chemistry - Question 3 - 2015 - Paper 1
Question 3
Ethane reacts with chlorine to form chloroethane.
C2H6(g) + Cl2(g) → C2H5Cl(g) + HCl(g)
(a) Use bond energies from the Data Booklet to calculate the enthalpy chang... show full transcript
Worked Solution & Example Answer:Ethane reacts with chlorine to form chloroethane - CIE - A-Level Chemistry - Question 3 - 2015 - Paper 1
Step 1
Use bond energies from the Data Booklet to calculate the enthalpy change for this reaction. Include a sign in your answer.
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Answer
To calculate the enthalpy change, we need to use the bond energies of the reactants and products:
Bond breaking:
C–H bonds in ethane (C2H6): There are 6 C–H bonds, contributing a total energy of 6imes410extkJ=2460extkJ.
Cl–Cl bond: 1 bond broken at 242 kJ.
Total energy for bond breaking = 2460+242=2702extkJ.
Bond forming:
C–Cl bond in chloroethane: 1 bond formed at 340 kJ.
This means the enthalpy change for the reaction is −1931extkJ/mol (exothermic).
Step 2
State the conditions needed for this reaction to occur.
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Answer
The reaction requires UV light or high temperature to initiate the chlorine radical formation.
Step 3
Use a series of equations to describe the mechanism of this reaction including the names of each stage and an indication of how butane can be produced as a minor by-product.
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Answer
The mechanism proceeds through three stages:
Initiation:
Cl2 → 2 Cl•
Propagation (two steps):
Step 1: C2H6 + Cl• → C2H5Cl + H•
Step 2: C2H5• + Cl2 → C2H5Cl + Cl•
Termination:
Two radicals can combine such as:
C2H5• + C2H5• → C4H10 (butane)
Thus, butane can be produced as a minor by-product during the termination phase.
Step 4
Give the name of X.
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Answer
The name of X is ethene.
Step 5
Suggest the reagent and conditions needed for reaction 1.
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The reagent for reaction 1 is KOH or NaOH, and the conditions required are heat and reflux.
Step 6
Suggest the reagent and conditions needed for reaction 2.
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The reagent for reaction 2 is H2, using either Pt or Ni as a catalyst.
